As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates (e.g., high-purity quartz or glass) that have an absorber layer (e.g., chrome) formed on a substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semiconductor wafers in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
Before fabricating production quality integrated circuits (ICs), semiconductor manufacturers typically test the circuit design by using a prototype photomask. The cost of manufacturing prototypes that include advanced designs (e.g., feature sizes with a dimension less than 0.18 microns) has become a concern to photomask manufacturers since the cost of manufacturing photomasks is rapidly increasing. One factor that effects the cost of manufacturing a prototype photomask is the cycle time needed to print an image of the IC in a resist layer of a photomask blank. Currently, multiple instances of an IC are placed on a single photomask and each instance must meet standards in a production specification associated with a specific manufacturing process. For example, if a photomask contains twenty die and only nineteen die meet the standards in the production specification, the entire photomask is rejected and the process for manufacturing the photomask design starts from the beginning.
The yield associated with the prototype photomasks may be increased by reducing the number of instances of the IC that are included on the photomask. In this example, the decrease in throughput caused by reducing the number of ICs formed on the photomask may be offset by the yield improvement and the cycle time gains in the mask manufacturing processes. All IC sites, however, must meet the standards in the production specifications and any photomasks that have at least one defective IC site are rejected.